Radiotherapy consists of projecting onto a predetermined region of a patient's body, a radiation beam so as to eliminate or reduce malignant or benign tumors existing therein as well as treating non-cancerous ailments. Such treatment is usually carried out periodically and repeatedly. At each medical intervention, the patient must be positioned with respect to the radiation source in order to irradiate the selected region with the highest possible accuracy to avoid radiating adjacent tissue on which radiation beams would be harmful. If movement of a patient is detected during treatment, the treatment should be halted to avoid irradiating areas of a patient other than the treatment location.
For this reason a number of monitoring systems for assisting the positioning of patients during radiotherapy and detecting patient movement have therefore been proposed such as those described in Vision RT's earlier patents and patent applications U.S. Pat. No. 7,889,906, U.S. Pat. No. 7,348,974, U.S. Pat. No. 8,135,201, US2015/062303 and US2015/0265852.
In the systems described in Vision RT's patent applications, images of a patient are obtained using a stereoscopic camera of a speckled pattern projected onto a portion of the patient being monitored. A model generation module determines transformations to identify and match corresponding portions of these images received by the left and right lenses of the stereoscopic camera. These matches are then utilized to generate a 3D model of the patient which is compared with a stored reference model of the surface of the patient when correctly positioned to confirm the patient is in the correct position. Typically such a comparison involves undertaking Procrustes analysis to determine a transformation which minimizes the differences in position between points on the surface of a patient identified by data generated based on live images and points on the stored reference model of the surface of a patient.
For stereotactic surgery, in particular when treating brain tumors, it is essential that the patient is positioned relative to the radiation delivery system with very high accuracy so that radiation is delivered to the tumor, and not the surrounding healthy tissue. For this reason, the head of a patient undergoing stereotactic surgery is securely attached to a couch via a patient restraint such as a head mask so that the patient cannot move their head during treatment. The same considerations apply in the case of other types and locations of tumor where alternative patient restraints are employed.
A patient restraint such as a mask is normally made out of a thermoplastic material which is heated prior to an initial treatment session. The mask is then molded to a patient's head by being placed over the patient's face and then allowed to set. The resultant head mask completely encloses a patient's head and thus restricts movement and allows a patient to be placed into a fixed position for each treatment session. Examples of such full head masks are disclosed in WO03/061477 and WO04/032781. Although a full-head mask is an improvement on immobilizing a patient solely with a chin strap, as the mask completely obscures a patient's face, it is not possible to monitor for movement of the face during treatment. This problem has been addressed by using an alternative head mask described U.S. Pat. No. 8,662,083 which includes an aperture arranged to leave a patient's face substantially free to allow for monitoring by a stereoscopic camera.
Improvements in patient monitoring systems are still desired.